A cycloidal planetary gear speed reducer of this type is known from the German patent DE 195 18 160 A1. In the cycloidal planetary gear speed reducer there are drive plates, instead of a sun gear, for driving the planet gears, where in this case the drive plates perform a circular translation with respect to the planet gears. The drawback with this design is the complicated and space intensive assembly. In addition, when the drive plates are executing a circular translation movement, an inaccurate engagement of these drive plates with the planet gears may lead to noises, an increase in friction and malfunctions.